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I was told by an old friend of mine that has been doing HVAC longer than me that you can place the coil on the return side of a furnace. I myself was always taught that they can only be installed on the supply side. Will it work properly if the coil is installed on the return side and if so in what circumstances or what kind of equpiment????

In Canada you can now put the coil in the return of a duel fuel system. In the past the thought was if it leaked and the burner was on it would send phosgene gas into the supply, for whatever reason the code has been changed. You can have a natural gas furnace and a heat pump, with the coil in the return, operate at the same time.

I can see how in heat mode a coil before the furnace would work fine, but the issues are the same in cooling mode with moisture on the heat exchanger. Most heat exchangers these days are luminized (don't know correct spelling) and I would think that should protect them somewhat from condensation damage. On an older furnce I am sure the exchanger would just rust like crazy.

I've seen a coil on the return on a furnace, it took out the heat exchanger in a little over a year. Could this be one of the main problems with package units heat exchangers since they are all draw thru systems?

Packaged gas heating and cooling.....also see a lot of commercial air conditioining with gas duct heaters....

Its hard to tell if the its the moisture from the ac or just the substandard quality of materials that causes the heat exchangers on these units to fail... One things for sure...when you see a package unit...it was designed by the manufacturer to be that way...its not some cobbled up mess that some slack jawed would be hvac engineer thought up to sell a job.....

Ive always been taught that humidifiers on returns of gas furnaces were bad too....but I see them all the time around here...steam type humidifiers on returns....I always tell the unlucky recipient of that mess that its wrong and that they need to have that exchanger checked at least once a year because of the fact....but after seeing so many furnaces with heat exchangers that have actually lasted for over a decade plus with such a set up I gave up on shutting off the humidifiers and starting a bees nest of trouble telling the customer that the humidifier installation would probably get them killed........just document it and go on.... Ive seen them on new installs too....passed by the local building inspector

I thought the condensation would form on the inside of the heat exchanger, not the outside, due to the flue being open to the outside air, why would a heat exchanger sweat with a coil in the return but not the blower or any other metal inside the cabinet? doesnt make sense